1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to the transmission and reception of control channels and data channels to and from, respectively, UEs with limited capabilities.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points such as Base Stations (BSs) (which may also be referred to as NodeBs) to User Equipments (UEs) and an UpLink (UL) that conveys transmission signals from UEs to reception points such as NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile, and may be a device such as a cellular phone, a personal computer device, etc. A NodeB, which is generally a fixed station, may also be referred to as an access point or other equivalent terminology.
DL signals include data signals, which carry information content, control signals, and Reference Signals (RSs), which are also known as pilot signals. A NodeB conveys data information to UEs through respective Physical Downlink Shared CHannels (PDSCHs) and control information through respective Physical Downlink Control CHannels (PDCCHs). Multiple RS types may be supported, such as a Common RS (CRS) transmitted over substantially the entire DL BandWidth (BW) and the DeModulation RS (DMRS) transmitted in a same BW as an associated PDSCH.
UL signals also include data signals, control signals, and RSs. UEs convey data information to NodeBs through respective Physical Uplink Shared CHannels (PUSCHs) and control information through respective Physical Uplink Control CHannels (PUCCHs). A UE transmitting data information may also convey control information through a PUSCH. The RS may be a DMRS or a Sounding RS (SRS), which a UE may transmit independently of a PUSCH.
Downlink Control Information (DCI) serves several purposes, and is conveyed through DCI formats in respective PDCCHs. For example, DCI includes DL Scheduling Assignments (SAs) for PDSCH reception and UL SAs for PUSCH transmissions. As PDCCHs are a major part of a total DL overhead, the resources required to transmit PDCCHs directly reduce DL throughput. One method for reducing PDCCH overhead is to scale it's the size of the overhead according to the resources required to transmit the DCI formats during a DL Transmission Time Interval (TTI). When Orthogonal Frequency Division Multiple (OFDM) is used as a DL transmission method, a Control Channel Format Indicator (CCFI) parameter transmitted through a Physical Control Format Indicator CHannel (PCFICH) can be used to indicate a number of OFDM symbols occupied by PDCCHs in a DL TTI.
FIG. 1 is a diagram illustrating a conventional structure for PDCCH transmissions in a DL TTI.
Referring to FIG. 1, a DL TTI includes one subframe having a number N of OFDM symbols. In the present example, N=14. A DL control region that includes PDCCH transmissions occupies a first M OFDM symbols 110. A remaining N-M OFDM symbols are used primarily for PDSCH transmissions 120. A PCFICH 130 is transmitted in some sub-carriers, also referred to as Resource Elements (REs), of a first OFDM symbol and includes 2 bits indicating a DL control region size of M=1, or M=2, or M=3 OFDM symbols. Moreover, some OFDM symbols also contain respective RS REs 140 and 150. These RSs 140 and 150 are transmitted substantially over an entire DL operating BandWidth (BW), and are referred to as Common RSs (CRSs), as they can be used by each UE to obtain a channel estimate for its DL channel medium and to perform other measurements. The BW unit for a PDSCH or a PUSCH over a subframe is referred to as a Physical Resource Block (PRB). A PRB includes several REs, such as 12 Res, for example.
A PDCCH and a PCFICH transmitted with the conventional structure in FIG. 1 are referred to as C-PDCCH and a C-PCFICH, respectively. Additional control channels may be transmitted in a DL control region but are not shown for brevity. For example, when using a Hybrid Automatic Repeat reQuest (HARM) process for a transmission of data Transport Blocks (TBs) in a PUSCH, a NodeB may transmit HARQ-ACKnowledgement (ACK) information in a Physical Hybrid-HARQ Indicator CHannel (PHICH) to indicate to a UE whether its previous transmission of each data Transport Block (TB) in a PUSCH was correctly received (i.e., through an ACK) or incorrectly received (i.e., through a Negative ACK (NACK)). A PHICH transmitted with a conventional structure is referred to as C-PHICH. The aforementioned conventional DL control channels will be jointly referred to as C-CCHs.
FIG. 2 is a diagram illustrating a conventional encoding and transmission process for a DCI format.
Referring to FIG. 2, a NodeB separately codes and transmits each DCI format in a respective PDCCH. A Radio Network Temporary Identifier (RNTI) for a UE, for which a DCI format is intended for, masks a Cyclic Redundancy Check (CRC) of a DCI format codeword in order to enable the UE to identify that a particular DCI format is intended for the UE. The CRC of (non-coded) DCI format bits 210 is computed using a CRC computation operation 220, and the CRC is then masked using an exclusive OR (XOR) operation 230 between CRC and RNTI bits 240. The XOR operation 230 is defined as: XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC bits are appended to DCI format information bits using a CRC append operation 250, channel coding is performed using a channel coding operation 260 (e.g. an operation using a convolutional code), followed by rate matching operation 270 applied to allocated resources, and finally, an interleaving and a modulation 280 operation are performed, and the output control signal 290 is transmitted. In the present example, both a CRC and a RNTI include 16 bits.
FIG. 3 is a diagram illustrating a conventional reception and decoding process for a DCI format.
Referring to FIG. 3, a UE receiver performs a reverse of the operations performed by the NodeB transmitter in order to determine whether the UE has a DCI format assignment in a DL subframe. A received control signal 310 is demodulated and the resulting bits are de-interleaved at operation 320, a rate matching applied at a NodeB transmitter is restored through operation 330, and data is subsequently decoded at operation 440. After decoding the data, DCI format information bits 360 are obtained after extracting CRC bits 350, which are then de-masked 370 by applying the XOR operation with a UE RNTI 380. Finally, a UE performs a CRC test 390. If the CRC test passes, a UE determines that a DCI format corresponding to the received control signal 310 is valid and determines parameters for signal reception or signal transmission. If the CRC test does not pass, a UE disregards the presumed DCI format.
The DCI format information bits correspond to several Information Elements (IEs) such as, for example, the Resource Allocation (RA) IE indicating the part of the DL BW or UL BW allocated to a UE for PDSCH reception or PUSCH transmission, respectively, the Modulation and Coding Scheme (MCS) IE indicating the data MCS, the Transmission Power Control (TPC) IE indicating an adjustment to the PUSCH transmission power or to the HARQ-ACK signal transmission power in a PUCCH, the New Data Indicator (NDI) IE informing a UE whether the scheduled data TB corresponds to a new transmission or to a retransmission for the same HARQ process, and so on.
In order to avoid a C-PDCCH transmission to a UE that blocks a C-PDCCH transmission to another UE, a location of each C-PDCCH in the time-frequency domain of a DL control region is not unique. Therefore, a UE may perform multiple decoding operations per DL subframe to determine whether there are any C-PDCCHs intended for the UE in a DL subframe. The REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of DCI format bits in FIG. 2, a number of CCEs for a respective C-PDCCH depends on a channel coding rate (Quadrature Phase Shift Keying (QPSK) is the modulation scheme in the present example). A NodeB may use a lower channel coding rate (i.e., more CCEs) for transmitting PDCCHs to UEs experiencing a low DL Signal-to-Interference and Noise Ratio (SINR) than to UEs experiencing a high DL SINR. The CCE aggregation levels may include, for example, 1, 2, 4, and 8 CCEs.
For a C-PDCCH decoding process, a UE may determine a search space for candidate C-PDCCH transmissions after the UE restores the CCEs in the logical domain according to a common set of CCEs for all UEs (i.e., a Common Search Space (CSS)) and according to a UE-dedicated set of CCEs (i.e., a UE-Dedicated Search Space (UE-DSS)). A CSS may include the first C CCEs in the logical domain. A UE-DSS may be determined according to a pseudo-random function having UE-common parameters as inputs, such as the subframe number or the total number of CCEs in the subframe, and UE-specific parameters such as the RNTI. For example, for CCE aggregation levels Lε{1, 2, 4, 8}, the CCEs corresponding to PDCCH candidate m are given by Equation (1)CCEs for C-PDCCH candidate m=L·{(Yk+m)mod └NCCE,k/L┘}+i.  (1)
In Equation (1), NCCE,k is the total number of CCEs in subframe k, i=0, . . . , L−1, m=0, . . . , MC(L)−1, and MC(L) is the number of C-PDCCH candidates to monitor in the search space. Exemplary values of MC(L) for Lε{1, 2, 4, 8} are, respectively, {6, 6, 2, 2}. For the UE-CSS, Yk=0. For the UE-DSS, Yk=(A·Yk-1) mod D, where Y−1=UE_RNTI≠0, A=39827, and D=65537.
DCI formats conveying information to multiple UEs are transmitted in a CSS. Additionally, if enough CCEs remain after the transmission of DCI formats conveying information to multiple UEs, a CSS may also convey some DCI formats for PDSCH reception or PUSCH transmission. A UE-DSS exclusively conveys DCI formats for PDSCH reception or PUSCH transmission. For example, a CSS may include 16 CCEs and support 2 DCI formats with L=8 CCEs, or 4 DCI formats with L=4 CCEs, or 1 DCI format with L=8 CCEs and 2 DCI formats with L=4 CCEs. The CCEs for a CSS are placed first in the logical domain (prior to interleaving).
FIG. 4 is a diagram illustrating a conventional transmission process of a DCI format in a respective C-PDCCH.
Referring to FIG. 4, after channel coding and rate matching are performed (as described with reference to FIG. 2), encoded DCI format bits are mapped to C-PDCCH CCEs in the logical domain. The first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are used for C-PDCCH transmission to UE1. The next 2 CCEs (L=2), CCE5 411 and CCE6 412, are used for C-PDCCH transmission to UE2. The next 2 CCEs (L=2), CCE7 421 and CCE8 422, are used for C-PDCCH transmission to UE3. Finally, the last CCE (L=1), CCE9 431, is used for C-PDCCH transmission to UE4.
The DCI format bits are then scrambled, at step 440, by a binary scrambling code, and the scrambled bits are modulated at step 450. Each CCE is further divided into Resource Element Groups (REGs). For example, a CCE including 36 REs can be divided into 9 REGs, such that each REG includes 4 REs. In step 460, interleaving is applied among REGs in blocks of four QPSK symbols. For example, a block interleaver may be used where interleaving is performed on symbol-quadruplets (i.e., four QPSK symbols corresponding to the four REs of a REG) instead of on individual bits. After interleaving the REGs, in step 470, a resulting series of QPSK symbols may be shifted by J symbols, and finally, in step 480, each QPSK symbol is mapped to an RE in a DL control region. Therefore, in addition to RSs from NodeB transmitter antennas 491 and 492, and other control channels, such as a PCFICH 493 and a PHICH (not shown), REs in a DL control region contain QPSK symbols for PDCCHs corresponding to DCI formats for UE1 494, UE2 495, UE3 496, and UE4 497.
The C-PDCCH structure in FIG. 4 uses a maximum of M=3 OFDM symbols and transmits the signal substantially over a total DL BW. As a consequence of using such a structure, such a control region has a limited capacity, and therefore cannot achieve interference coordination in the frequency domain. There are several cases where expanded capacity or interference coordination in the frequency domain is needed for transmission of control signals. One such case is the extensive use of spatial multiplexing for PDSCH transmissions, where multiple DL SAs correspond to the same PDSCH resources. Another case is for heterogeneous networks where DL transmissions in a first cell experience strong interference from DL transmissions in a second cell and DL interference coordination in the frequency domain between the two cells is needed.
Due to REG-based transmission and interleaving of C-PDCCHs, the control region cannot be expanded to include more OFDM symbols while maintaining compatible operation with existing UEs that cannot be aware of such an expansion. An alternative to the REG-based transmission and interleaving of C-PDCCHs is to extend the control region in the PDSCH region and use individual PRBs for transmitting new PDCCHs, which are referred to as Enhanced PDCCHs (E-PCCCHs).
FIG. 5 is a diagram illustrating a conventional E-PDCCH transmission structure.
Referring to FIG. 5, although E-PDCCH transmissions start immediately after C-PDCCH transmissions 510 and are transmitted over all remaining DL subframe symbols, they may instead always start at a fixed location, such as the fourth OFDM symbol. E-PDCCH transmissions occur in four PRBs, 620, 630, 640, and 650, while remaining PRBs 660, 662, 664, 666, 668 are used for PDSCH transmissions. As an E-PDCCH transmission over a given number of subframe symbols may require fewer REs than the number of subframe symbols available in a PRB, multiple E-PDCCHs may be multiplexed in a same PRB. The multiplexing can be in any combination of possible domains (i.e., time domain, frequency domain, or spatial domain) and, in a manner similar to a PDCCH, an E-PDCCH includes at least one Enhanced CCE (E-CCE).
A transmission for an extended control channel (E-PDCCH, E-PCFICH, E-PHICH) may be in a same PRB, in which case the transmission is referred to as localized, or over multiple PRBs, in which case the transmission is referred to as distributed. The aforementioned Enhanced Control CHannels are jointly referred to as E-CCHs. The demodulation of information in an E-CCH may be based on a CRS or on a DMRS.
FIG. 6 is a diagram illustrating a conventional DMRS structure.
Referring to FIG. 6, DMRS REs 610 are placed in some OFDM symbols of a PRB. When there are two NodeB transmitter antenna ports, a first DMRS transmission is assumed to apply the Orthogonal Covering Code (OCC) of {1, 1} over two DMRS REs that are located in a same frequency position and are successive in the time domain while a second DMRS transmission is assumed to apply the OCC of {1, −1}. A UE receiver can estimate the channel experienced by the signal from each NodeB transmitter antenna port by removing a respective OCC.
UL Control Information (UCI) is transmitted from a UE to a NodeB to facilitate PDSCH transmissions or PUSCH transmissions. UCI includes HARQ-ACK information associated with a transmission of one or more Transport Blocks (TBs) in a PDSCH, Channel State Information (CSI) informing a NodeB about a channel experienced by DL transmissions to a UE, and Service Request (SR) informing a NodeB that a UE has data to transmit. CSI may include a Channel Quality Indicator (CQI), which implicitly or explicitly informs a NodeB of a wideband or a sub-band SINR experienced by a UE, a Precoding Matrix Indicator (PMI), which informs of an entry in a precoding matrix for a NodeB to apply beamforming to a DL signal transmission, or a Rank Indicator (RI) which informs a NodeB that a UE can support spatial multiplexing for a DL signal transmission.
FIG. 7 is a diagram illustrating a conventional structure for a HARQ-ACK signal transmission in one of the two subframe slots of a PUCCH.
Referring to FIG. 7, HARQ-ACK signals and RS that enable coherent demodulation of HARQ-ACK signals are transmitted in one slot 710 of a PUCCH subframe including 2 slots. The transmission in the other slot can be at a different part of the UL BW. HARQ-ACK information bits 720 modulate 730 a Zadoff-Chu (ZC) sequence 740, for example using Binary Phase Shift Keying (BPSK) for 1 HARQ-ACK bit or QPSK for 2 HARQ-ACK bits, which is then transmitted after performing a Inverse Fast Fourier Transform (IFFT) operation 750. Each RS 760 is transmitted using an unmodulated ZC sequence.
For a UL system BW including NRBmax,UL RBs, where each RB includes NscRB=12 REs, a ZC sequence ru,v(α)(n) can be defined by a Cyclic Shift (CS) α of a base ZC sequence ru,v(n) according to ru,v(α)(n)=ejαnru,v(n), 0≦n<MscRS, where MscRS=mNscRB is a length of the ZC sequence, 1≦m≦NRBmax,UL, and ru,v(n)=xq (n mod NZCRS) where a qth root ZC sequence is defined by
                    x        q            ⁡              (        m        )              =          exp      ⁡              (                                            -              j                        ⁢                                                  ⁢            π            ⁢                                                  ⁢                          qm              ⁡                              (                                  m                  +                  1                                )                                                          N            ZC            RS                          )              ,      0    ≤    m    ≤                  N        ZC        RS            -      1      with q given by q=└q+½┘+v·(−1)└2q┘ and q given by q=NZCRS·(u+1)/31. A length NZCRS of a ZC sequence is given by a largest prime number such that NZCRS<MscRS. Multiple RS sequences can be defined from a single base sequence through different values of α. A PUCCH transmission is assumed to be in one RB (MscRS=NscRB).
FIG. 8 is a diagram illustrating a conventional structure for a periodic CSI signal transmission in one of the two subframe slots of a PUCCH.
Referring to FIG. 8, CSI signals and RSs that enable coherent demodulation of CSI signals are transmitted in one slot 810 of a PUCCH subframe including 2 slots. The transmission in the other slot can be at a different part of the UL BW. After encoding (using for example a block code) and modulation (using for example QPSK) which are not shown for brevity, encoded CSI bits 820 modulate 830 a ZC sequence 840 which is then transmitted after performing an IFFT operation 840. Each RS 850 is transmitted using an unmodulated ZC sequence.
FIG. 9 is a diagram illustrating a transmitter for a ZC sequence for which, without modulation, serves as an RS and with modulation serves as a HARQ-ACK signal or as a CSI signal.
Referring to FIG. 9, a mapper 920 maps a ZC sequence 910 to REs of an assigned transmission BW as they are indicated by RE selection unit 925. Subsequently, an IFFT is performed by IFFT unit 930, a CS is applied to the output by CS unit 940, followed by scrambling with a cell-specific sequence using scrambler 950, a Cyclic Prefix (CP) is inserted by CP insertion unit 960, and the resulting signal is filtered by filter 970. Finally, a transmission power PPUCCH is applied by power amplifier 980 and a ZC sequence is transmitted 990. The reverse of these operations is performed at a NodeB receiver.
Different CSs of a ZC sequence provide orthogonal ZC sequences. Therefore, different CSs a of a same ZC sequence can be allocated to different UEs in a same PUCCH RB and achieve orthogonal multiplexing for HARQ-ACK signals and RS or for CSI signals and RS. For a RB including NscRB=12 REs, there are 12 different CSs. A number of usable CSs depends on the channel dispersion characteristics, and can typically range between 3 and 12 CSs. Orthogonal multiplexing can also be in the time domain using OCC where PUCCH symbols conveying a same signal type in each slot are multiplied with elements of an OCC. For example, for the structure in FIG. 7, a HARQ-ACK signal transmission can be modulated by a length-4 OCC, such as a Walsh-Hadamard (WH) OCC, while an RS transmission can be modulated by a length-3 OCC, such as a DFT OCC. In this manner, the multiplexing capacity is increased by a factor of 3 (determined by the OCC with the smaller length Noc).
UEs may communicate over a total system BW or over only a part of the system BW. The former UEs can benefit from most or all network capabilities for PDSCH receptions or PUSCH transmissions, are typically used by humans, and are referred to as conventional UEs herein. The latter UEs have substantially reduced capabilities compared to the former UEs in order to substantially reduce their cost, are typically associated with machines, and are referred to as Machine Type Communication (MTC) UEs herein.
MTC UEs are low cost devices targeting various low data rate traffic applications including smart metering, intelligent transport systems, consumer electronics, and medical devices. Typical traffic patterns from MTC UEs are characterized by low duty cycles and small data packets in the order of a few tens or a few hundred bytes. MTC UEs have typically low mobility, but high mobility MTCs, such as in motor vehicles, for example, may also exist. Also, unlike conventional UEs, MTC UEs generate more UL than DL traffic and a majority of DL traffic is higher layer control information, such as Radio Resource Control (RRC) information, for configuration of a communication with a NodeB.
Unlike conventional UEs, such as for example a smart-phone, which may have many features, MTC UEs only have a minimum of necessary features, and the modem is a primary contributor to the cost of an MTC UE. Therefore, main cost drivers for MTC UEs are Radio Frequency (RF) components and Digital Base-Band (DBB) components mainly for the receiver. RF components include the power amplifier, filters, transceiver radio chains, and possibly a duplexer (for full duplex FDD operation). DBB components include a channel estimator, a channel equalizer, a PDCCH decoder, a PDSCH decoder, and a subframe buffer. For example, a channel estimator may be based on a Minimum Mean Square Error (MMSE) estimator, a channel equalizer may be an FFT, a PDCCH decoder may be a decoder for a Tail Biting Convolutional Code (TBCC), and the data decoder may be a decoder for a Turbo Code (TC) or a TBCC.
RF costs are related to implementation and production methods as well as to design choices. For example, considering economies of scale, it may be more cost effective to use the same amplifier for conventional UEs and MTC UEs, which will also ensure a same UL coverage, while the number of transmitter antennas for MTC UEs may be limited to one antenna.
DBB costs are related to the communication capabilities of MTC UEs and are dominated by the receiver complexity, which is typically about an order of magnitude larger than the transmitter complexity. As the channel estimator complexity, the FFT complexity and the subframe buffering requirements are directly associated to the reception BW, DL transmissions to MTC UEs may be over a smaller BW than DL transmissions to conventional UEs. For example, DL transmissions to MTC UEs may be over a 1.4 MHz BW while DL transmissions to conventional UEs may be over a 20 MHz BW.
A complexity of a PDCCH decoder depends on a number of decoding operations an MTC UE needs to perform per DL subframe. As MTC UEs do not need to support a same number of transmission modes (TMs) as conventional UEs, for example MTC UEs may not need to support spatial multiplexing for PDSCH or for PUSCH, a maximum number of decoding operations per DL subframe can be significantly smaller than that for conventional UEs. A complexity of a PDSCH decoder depends on a maximum supportable data rate. Allowing for a relatively small maximum data rate for MTC UEs provides a limit to an associated decoder complexity.
MTC UEs generally access the communication system in the same manner as conventional UEs. Synchronization signals are first acquired to establish synchronization with a NodeB followed by a detection of a Broadcast CHannel (BCH) that conveys essential information for subsequent communication between a NodeB and UEs (conventional or MTC ones). Regardless of a DL BW, synchronization signals and BCH are transmitted over a minimum DL BW located in the center of a DL BW, such as in the middle six RBs of a DL BW, and over a number of OFDM symbols in a subframe, for example. After establishing communication with a NodeB, a different part of a DL BW may be allocated to an MTC UE.
One aspect of supporting communication of MTC UEs is a design of DL control signaling. As transmissions of C-CCHs are distributed substantially over a total DL system BW, if MTC UEs receive DL transmissions only in a BW smaller than the total BW, the MTC UEs may not be able to decode C-CCHs. The use of E-CCHs can provide DL control signaling support for MTC UEs, but due to reduced receiver DBB capabilities of MTC UEs, it may not be possible to use a same design of E-CCHs for MTC UEs and for conventional UEs.
Another aspect of supporting communication of MTC UEs is a reduction in an overhead associated with DL control signaling and UL control signaling for MTC UEs. As data TBs associated with MTC UEs are typically substantially smaller than the ones associated with conventional UEs, applying same DL or UL control or data multiplexing mechanisms for MTC UEs as for conventional UEs will cause resource utilization for MTC UEs to be substantially worse than the resource utilization for conventional UEs.
Another aspect of supporting communication of MTC UEs is applying a set of functionalities associated with signal transmissions to or from MTC UEs. If signal transmissions for MTC UEs are over a smaller BW than for conventional UEs, different functionalities can be often needed for MTC UEs compared to conventional UEs. Higher power utilization can also be desirable for MTC UEs.
Therefore, there is a need to design control signaling and data or control multiplexing for MTC UEs.
There is also a need to reduce control overhead for MTC UEs.
There is also a need to define, whenever necessary, different functionalities for signal transmissions to or from MTC UEs compared to conventional UEs and to increase power savings for MTC UEs.